回答編集履歴
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Dictionary(grouping:)
test
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@@ -2,7 +2,7 @@
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let array = ["A", "B", "B", "B", "C", "D", "D"]
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print(
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print(array.reduce(into: [String:Int]()) {$0[$1] = ($0[$1] ?? 0) + 1}.filter {$0.1 > 1}.keys)
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//=> ["B", "D"]
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@@ -11,3 +11,23 @@
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と、考えてから**daisuke7**さんの回答に気付く‥。
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##### Dictionary(grouping:)使用
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さっそく吸収ｗ
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```swift
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print(Dictionary(grouping: array) {$0}.filter {$0.value.count > 1}.keys)
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```
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これくらいのシンプルさなら（見た目的に）大丈夫？
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